1. Field of the Invention
The present invention relates to a method and apparatus for connecting branch networks with a trunk network having duplex optical transmission lines in an optical transmission system.
2. Description of the Related Art
Recently, a large-scale local area network (LAN) has been developed. As shown in FIG. 1, ring type LAN 10 having duplex optical transmission lines L1 and L2 is often used as a trunk network. Bus type LANs 30a, 30b, 30c, and 30d connected with ring type LAN 10 through nodes 20a, 20b, and 20c having a loop back (U-link) function are often used as branch networks. Note that nodes 20a, 20b, and 20c have transmitting and receiving units 21a, 21b, and 21c, and LAN interfaces 22b, 23b, 24a, and 24b for performing data transfer between ring type LAN 10 and bus type LANs 30a, 30b, 30c, and 30d. More specifically, as shown in FIG. 2, transmitting and receiving unit 21a of node 20a has optical transmitting and receiving circuits 10a and 10b, switching circuit 11, serial-parallel converter 12, and network controller 13. Optical signals on optical transmission lines L1 and L2 are converted to electric signals by optical transmitting and receiving circuits 10a and 10b, and are input to serial-parallel converter 12 through switching circuit 11 controlled by network controller 13. Serial-parallel converter 12 converts the input electric signals into parallel data and outputs the parallel data onto internal bus 14.
In this arrangement, when optical transmission line L1 is disconnected, switching circuit 11 is switched by network controller 13, and only the electric signal from optical transmitting and receiving circuit 10a is input to serial-parallel converter 12 through switching circuit 11.
A ring type LAN is not used as a branch network since two sets of expensive optical transmitting and receiving circuits are necessary at the nodes of the ring type LAN, as shown in FIG. 2. In place of adding new nodes, repeaters 32a to 32d in LAN 32 are arranged through LAN interface 23a, thereby extending transmission lines to desired locations. However, an extended distance is limited by a data transmission delay time caused by extension of the transmission lines.
Star type LAN 33 is connected with ring type LAN 10 through LAN interface 22a. However, optical star coupler 33a or the like used in star type LAN 33 is expensive, and an extended distance is also limited by a data transmission delay time caused by extension of the transmission lines. Therefore, star type LAN 33 is not used except for a special application.
As described above, a strong demand has arisen for an inexpensive apparatus which can facilitate connection of bus type branch networks with a ring type trunk network.